Multi-channel seismic and flexural analysis of the Westland sedimentary basin, South Island, New Zealand

Abstract

In the summer of 1996 a multi-channel marine seismic line (line 2W) was acquired off the west coast of South Island, New Zealand as part of the SIGHT {South Island GeopHysical Transect) campaign. Running perpendicular to the coast, line 2W is approximately 200 kilometres in length, terminating within 25 kilometres of the Alpine Fault. The line cuts across the southern end of a sedimentary basin loosely termed the Westland Sedimentary Basin. This thesis presents the processing and interpretation of line 2W with particular attention focused upon the sedimentary sequence that records deformation resulting from the continent-continent collision zone. Dating of prominent seismic reflectors within line 2W is achieved through the use of open-file industry seismic and well exploration data. Pre-Miocene sediments are observed to thicken to the west, a trend that is reversed in Post-Miocene sediments. This reversal in thickening direction indicates an abrupt change in the tectonic driving force for the Westland Sedimentary Basin. From the Miocene-Pliocene boundary the basin starts to be depressed by a driving force to the east, the timing of which is consistent with the initiation of an increased convergent component across the Australian-Pacific plate boundary and the associated rapid rise of the Southern Alps. Within line 2W sedimentary horizons for the Tertiary show a pronounced downward deflection on the order of 2-3 kilometres towards the Southern Alps. Numerical modelling shows that this deflection is too large to be due to sediment loading at a passive margin and instead it is proposed that the edge of the Australian plate is carrying part of the imposed load of the Southern Alps. Three-dimensional flexure modelling yields an estimate of 15 km ±5 km for the effective elastic thickness (Te) of the Australian plate but highlights the need for in-plane stresses to produce the observed deformation of the sedimentary horizons. Such a low value of Te may be ascribed to the high lithospheric curvature and strain rates associated with the Westland Sedimentary Basin, crust-mantle decoupling, inherited weakness in the upper crust, high heat flow, or a combination of the above. The flexural models presented here predict a significant proportion of the Southern Alps are required as the load to produce the observed deflection of the Australian plate. Thus this study provides further evidence for an origin of the Westland Sedimentary Basin as a retroarc foreland basin. Moreover, the observation that the deepest part of the basin is adjacent to the highest part of the Southern Alps is in accord with the growth of the Southern Alps being the principal driving force for the basin.